In various digital systems, signals can be transmitted from a transmitter to a receiver via a transmission channel. The transmission channel may be any suitable wired (or wireless) medium which links the transmitter to the receiver. However, in many instances (e.g., high data transmission speeds), the transmission channel becomes lossy. The transmission losses can be a result of, among other things, interference, attenuation, and delay in the channel. Further, such losses can also have considerable detrimental effect on the transmitted signal by the time it reaches the receiver. For example, sufficient amplitude and phase distortion of the transmitted signal may result in inter-symbol interference (ISI) in the signal received at the receiver. ISI generally refers to the ‘smearing’ of a pulse or other symbol representing the logic state of one data bit to the degree such that it contributes to the content of one or more of the preceding (i.e., pre-cursor ISI) or succeeding (i.e., post-cursor ISI) data bits.
To guard against such detrimental effects, many serial receiver systems perform decision feedback equalization (DFE) on the received data. Such serial receiver systems may include (i) an analog front end that provides some continuous time linear equalization (CTLE), (ii) a variable gain amplifier (VGA) (iii) a sampler, a (iv) DFE that uses the quantized receive data to adaptively feedback a correction signal, and (v) a timing recovery unit. The DFE adapts its feedback to minimize the error of the sampled signal at the center of the data bit. The sampler includes an error sampler that is used to detect this error signal. Generally, there is only one error sampler, and this error sampler has a positive threshold. These error samples (along with the data samples) are then de-serialized by a significant factor and provided to the DFE. At the DFE, digital logic computes an error correction signal (e.g., “+1,” “0,” or “−1”) for each of the de-serialized error samples. After which, voting is applied to the set of error correction signals corresponding to the de-serialized word (i.e., combination of the de-serialized data bits) to generate a single error correction signal. Filtering is then applied to the generated single error correction signal to compute a digital tap weight for the DFE. This DFE tap weight may be fed back to the analog front end to perform a corrective operation on the incoming signal.
Most DFE-based receiver systems are designed with the assumption that the transmission channel is long. Further, for many of these channels, the attenuation is generally between 20-30 dB. As such, to compensate for the channel's attenuation, the transmitted signals are usually defined with large launch amplitudes. Although this makes sense for longer channels, in many situations the channel can be quite short or even non-existent. For example, many mobile applications use short channels due to the small feature size of hand-held devices (whereas many server applications have lossy channels due to the long circuit board traces and backplanes). Further, devices that use cables for communications, e.g., USB, must support both (i) lossy channels (i.e., due to long cables) and (ii) short channels (e.g., thumb drives). Most systems are designed to automatically adapt to a wide range of channels. With shorter (i.e., low loss) channels, the amplitude of the signal received at the receiver system will likely be larger than if the signal was transmitted via a longer channel. Large-amplitude signals can present a variety of problems for receiver systems that were originally designed to handle smaller amplitude ranges. For example, in order for the DFE feedback to operate properly, VGAs in DFE-based receivers have to adjust the signal's amplitude within a certain range. However, for large signals, the VGA will likely be unable to bring the signal amplitude within the appropriate operating range. Generally, if the amplitude of the signal is larger than the positive error threshold, the VGA will be unable to bring the signal within the appropriate operating range. Further, when the error samples for the positive data bits of the signal are predominately above the positive error threshold for a period of time, there will be no way for the DFE to determine the amount of distortion caused by the ISI. In this case, the DFE tap weights will randomly change based on the data pattern or, in some cases, drift with a negative bias causing the DFE weights to be large in magnitude and negative in sign, resulting in severe over-equalization. Although this severe over-equalization may have a small effect on the eye height, it may have a large, detrimental effect on the eye width at the receiver and also cause the timing recovery unit to lock to a non-optimal phase.
In order to address such problems, previous receiver systems included attenuators in the receive path. Similar to the attenuation in long channels, the attenuators attenuated the signal such that the VGA was able to bring the signal within the appropriate operating range. However, in many instances, the amplitude of the signal is still too large that the attenuation provided by the attenuator is not enough. Further, attenuators must be adaptively switched out for lossy channels to maintain an adequate signal. However, even with the adaption, the attenuators are complicated to control and require switches that distort the signal.
Accordingly, there is a need to prevent mis-equalization of signals transmitted over short transmission channels.